The present invention relates to a method and an apparatus for charging one or more electric vehicles.
Electric vehicles are being developed as an alternative to vehicles powered by petroleum based fuels such as gasoline. Advantages of electrically powered vehicles include the lack of exhaust pollutants emitted during the combustion process as well as a reduction in noise.
As is well known, one of the significant drawbacks to an electric vehicle is the storage capabilities of on-board batteries used to power the vehicle. Typical commuter electric vehicles have on-board batteries that have a capacity of 10 kW-hr to 25 kW-hr. This amount of energy will allow the vehicle to operate for a limited period of time, depending on the terrain, the speed of operation and the number of miles traveled before the batteries must again be recharged. It is believed that buses or other larger vehicles will have to be charged on a daily basis, if not more frequently.
Many modern electric vehicle battery charging systems are typically self-contained units for charging a single vehicle. A typical prior art charger system includes a utility interface having over-current protection, EMI filtering and lighting protection. Preferably, a high power factor rectifier and a low pass filter convert alternating current utility power to a direct current (DC). A switch-mode DC-AC inverter operating at high radio frequencies (RF) and a RF rectifier and another low pass filter transfer power to a device connectable to the vehicle battery. Overall control of the battery charging system is performed on the vehicle, although it may also be controlled off the vehicle. A battery management system sends signals to an off-board charger to adjust the current delivered to the battery or batteries. Complex charge algorithms are used in order to maximize efficiency and fully charge the batteries. A controller in the off-board charger receives the signals from the battery management system and sends signals to the power electronics in order to respond to the demand of the battery management system.
The components of the electric vehicle battery charging system can be located on or off the vehicle. At some point, the power must be coupled to the vehicle in order to charge the batteries. This coupling is performed by an interface device that determines which components of the charging system are on the vehicle and which are off the vehicle. The Society of Automotive Engineers (SAE) has advanced three well known techniques for electric vehicle charging systems. These techniques are differentiated from each other by the physical and electrical characteristics used to couple the charging system to the vehicle and transfer power to the vehicle battery. The three techniques include AC conductive coupling, DC conductive coupling and inductive coupling.
Electric vehicle battery charging can be done overnight with equalization or rapidly without equalization. "Equalization" ensures each battery of a plurality of batteries connected in series retains approximately the same charge. To equalize the batteries in an electric vehicle, energy must be provided in relatively small amounts (typically 1 kW or less) and for an extended time (typically four or more hours). This charging technique fully charges each of the batteries and has been shown to increase the useful life of the batteries. During rapid charging, power is provided to the batteries at the maximum rate allowed. Total power transfer can be in the range of 25 kW to 300 kW; however, the charging time is typically one half hour or less.
Even with rapid charging, there is still a well established need for slow or overnight charging, for example, when electric energy rates are lower. One method for charging a fleet of vehicles would be to have one slow charger per electric vehicle and one or a few separate rapid chargers. The slow chargers are used to charge the entire fleet overnight, while the rapid chargers would be used during the day as needed and as available in order to rapidly charge a vehicle from the fleet.
Since it is common practice for the on-board controller to control the charging process between the battery charger and the electric vehicle, the battery charger must be able to operate over a wide range of power transfer operating points. This requirement can cause significant losses in the power transfer path. For instance, in the case of an inductively coupled charger that operates according to SAE (Society of Automotive Engineers) standard J-1773, the switching frequency of the charger is modulated to change the output power delivered to the vehicle. Such chargers have a power stage that is a series resonant converter that operates above resonance. As the operating frequency is increased above resonance, the power delivered decreases. This control method provides one of the most common means of controlling the converter.
There is an ongoing need to have a charger that can charge a number of vehicles, each having its own maximum power transfer rating. Each vehicle would then control the charger and request power at its own safe level. Some vehicles can, and expect, to receive power rapidly (for example, 50 kW) while others can only handle power at relatively low charge rates (for example, 6.6 kW). As described above, an inductive battery charger can increase its operating frequency in order to lower the available output power. Unfortunately, the losses in many components on the vehicle are frequency dependent and the vehicle which operates fine at a given power level at a selected operating frequency may fail if operated at the same power level at a significantly increased operating frequency. The losses in the on-board components such as diodes, capacitors, copper conductors and couplers may be proportional to the operating frequency for a fixed output power. In addition, insufficient cooling is typically common in most existing designs (developed for low power charging (6.6 kW maximum)), thus causing extra thermal stress on components which can lead to failure if the operating frequency is increased for a given output power.